docs: def export(self, nidm_version, export_dir):
          """
          Create prov entities and activities.
          """
          self.add_attributes({
              PROV['type']: self.type,
              NIDM_DIMENSIONS_IN_VOXELS: json.dumps(self.dimensions.tolist()),
              NIDM_NUMBER_OF_DIMENSIONS: self.number_of_dimensions,
              NIDM_VOXEL_TO_WORLD_MAPPING:
              json.dumps(self.voxel_to_world.tolist()),
              NIDM_IN_WORLD_COORDINATE_SYSTEM: self.coordinate_system,
              NIDM_VOXEL_UNITS: json.dumps(self.units),
              NIDM_VOXEL_SIZE: json.dumps(self.voxel_size.tolist()),
              PROV['label']: self.label})